Hoolock gibbons  (Hoolock hoolock and Hoolock leuconedys)

Hylobates means ‘dweller in trees' and, as one of the most agile animals of the forests, these gibbons certainly suited their old name (4). Though other apes have long limbs, mobile shoulders and are able to stand erect, gibbons have developed even greater propulsive abilities in their upper arms, and are able to cover three meters in a single swing (2) (4). These species are two of the largest of the gibbons, and, like other members of the Hylobatidae family, males and females have different coat colours. Adult males have a completely black coat, while females are golden in colour, with darker brown cheeks (2). Both males and females have dark faces and, as their other common name (white-browed gibbon) suggests, bear white, curved brow streaks above the eyes. They also have longer body hair than other gibbons and, like all apes, lack tails (2). At birth, infants have a grey-white coat with a yellow tinge, which darkens as they mature (2).

It is not only appearances that distinguish gibbons in the forest. Their loud calls are unique and beautiful, with variations between species and even between sexes of a species. Male hoolocks emit two-phased calls that accelerate, while females reply with similar calls, but of a lower pitch (2) (5).